1. Field of the Invention
The present invention relates to a multicarrier transmission method and apparatus, and more particularly, to a method of suppressing a peak of a multicarrier transmitted signal, multicarrier transmission signal generating circuit with peak suppressing function, adaptive peak limiter, baseband signal processing LSI and multicarrier transmission apparatus.
2. Description of Related Art
In the field of mobile communications, for example, with respect to technical specifications in the W-CDMA (Wideband-Code Division Multiple Access) system, 3GPP has standardized. In the technical specifications, in addition to basic reception techniques (for example, Rake combining) that take advantages of CDMA, High Speed Downlink Packet Access (hereinafter, also abbreviated as HSDPA) with a faster rate of 10 Mbps has been also standardized.
HSDPA is a technique where a plurality of users that perform downlink packet transmission shares a downlink channel, radio channel quality of each user is checked, and optimal base stations transmit signals to respective users, thus improving the transmission efficiency. Using the technique achieves a transmission rate of 10 Mbps using a frequency bandwidth of 5 MHz.
Among specific techniques used in HSDPA are adaptive modulation that is a scheme for varying a modulation scheme and coding scheme corresponding to propagation environments and using M-ary modulation such as 16 QAM and 64 QAM suitable for large-capacity transmission, HARQ (Hybrid ARQ) that synthesizes a retransmission signal to improve the reception quality, and FCS (Fast Cell Selection) that achieves efficient packet transmission from a plurality of base stations.
In adaptive modulation, a base station transmits signals with M-ary modulation such as 16 QAM and 64 QAM and high-rate coding with a coding rate of, for example, ¾ when the reception quality in a mobile station is good, while transmitting signals with QPSK and low-rate coding with a coding rate of, for example, ¼ when the reception quality in a mobile station is poor.
In HSDPA, since QAM is used as a modulation scheme, with respect to signal quality in a band (Peak Code Domain Error (PCDE) and Error Vector Magnitude (EVM)), it is mandatory to comply with performance standard (TS25.141 Rel. 5) stricter than performance standard (TS25.141 Rel. 99) for general third-generation base station apparatus.
Meanwhile, the CDMA system has a significant feature of implementing concurrent communications by multiplexing user signals. For example, it is assumed that a frequency band assigned to a company permitted to locate base stations includes four channels (with carrier frequencies f1 to f4 respectively).
In this case, data of a plurality of users is multiplexed on one channel, and signals of the channels are transmitted at the same time from a shared antenna. In other words, four carriers, f1 to f4, are concurrently transmitted (multicarrier transmission).
When the multicarrier transmission is performed, a high-frequency amplifier provided at a last stage of a transmitter undergoes a heave load, and is required to secure the linearity in a wide band.
In order to reduce the load on the high-frequency amplifier, using a peak limiter, the processing for suppressing an instantaneous peak is performed on a baseband signal for multicarrier transmission.
The peak limiter is described in, for example, Japan Laid-Open Patent Publication Nos. 2002-164799 and 2002-44054.
However, conventional techniques related to the peak limiter have no consideration on High Speed Downlink Packet Access (HSDPA).
HSDPA is an advanced technique with a considerable amount of difficulty to practically implement, in any theory.
Further, as described above, since QAM is used as a modulation scheme, with respect -to signal quality in a band (PCDE and EVM), HSDPA needs to meet performance standard (TS25.141 Rel.5) stricter than performance standard (TS25.141 Rel.99) for general third-generation base station apparatus.
In mobile communication apparatuses such as cellular telephones, there have been severe demands always for cost reduction, miniaturization and low power consumption.
It is difficult for conventional techniques to implement HSDPA under various constrains imposed on mobile communication apparatuses.
For example, in HSDPA, the modulation scheme is varied corresponding to the channel quality. In this case, when characteristics of a peak limiter are adapted to 64 QAM with the strictest conditions as a reference and peak suppression is reduced, since the suppression on an instantaneous peak is not sufficient, and as a result, the load on a high-frequency amplifier in a subsequent stage is increased and the power efficiency in the high-frequency amplifier deteriorates. Meanwhile, when the peak suppression is enhanced, a signal loss degrades the signal quality.
In order to solve the problem, it is necessary to use an amplifier with high performance that secures the linearity in an extremely wide range. However, such a high-frequency amplifier is expensive, and becomes a significant obstacle in cost.
The problem is described above referring to W-CDMA communications as an example, but there is a possibility such a problem occurs in other communication systems (such as other CDMA systems) that perform high-speed packet transmission.